Real-time symmetrical modeling with distinct objects in Blender
This is what Groups and Group Instances are for, you just don't add modifiers to them.



Create your selection of objects you want to belong to this symmetry group, then group them together with Ctrl + G. Give it a descriptive name

Now from the Add menu add a new group instance with the previously given name. Scale it a factor of -1 in X so it becomes mirrored.

You may optionally want to make the instance non selectable in the Outliner so it doesn't accidentally get in the way.

Now freely work on your original group of objects and see them symetrized automatically. Whenever you feel like adding a new one just link it to the existing group.

You can do this by selecting any new objects, then Shift selecting one already belonging to the group so it remains the active one. Then press Ctrl + L > Make Links > Group.